Preliminary data suggest that the consequences of gestational x-irradiation on the central nervous system of the rat are expressed in morphology and function in ways which depend on the age of the animal. A longitudinal study is proposed of neuronal and bahavioral development in rats following prenatal x-irradiation with 125 R, 100 R, 75 R, and 50 R. A battery of tests of behavioral development will be used from birth to weaning (including righting reflex, acoustic startle and spatial maze exploration). Activity will be monitored up to 9 months. Neuronal differentiation in cerebral cortex, ectopic cortex (present at 125 R and possibly at lower doses), caudate nucleus and auditory cortex will be followed with the light microscope over the same preweaning and adult periods. Synaptogenesis will be followed with electron microscopy. Chicks will be compared with rats as a specie in which maternal effects are excluded during embryonic x-irradiation. It is proposed that the dose-response data obtained in these experiments will be useful in establishing the relative sensitivity of the different behavioral and morphological parameters to the toxic effects of x-irradiation.